Modal Identification of Over-Damped Structural Systems Using Extended Ibrahim Time-Domain Method

  • Chang-Sheng LinEmail author
  • Tse-Chuan Tseng
Conference paper
Part of the Conference Proceedings of the Society for Experimental Mechanics Series book series (CPSEMS)


In the previous study, the conventional Ibrahim time-domain method (ITD) using free-decay responses of structures has been extensively used in the modal-identification analysis, however, which is only applicable to identify the modal parameters of an under-damped structure. In the present paper, we propose a theoretical modification for ITD method, and extend the ITD method for modal identification of over-damped structural systems. The eigenvectors of the system matrix used in extended ITD method corresponding to the vibrating modes of a structural system in pair are sorted through the assurance index proposed in this paper based on the theory of structural dynamics. Numerical simulations confirm the validity of the proposed method for modal identification of over-damped structural systems.


Extended Ibrahim time-domain modal-identification method Over-damped structural systems Identification of modal parameter 



This research was supported in part of the financial funding provided by National Synchrotron Research Center (NSRRC). The first author would like to thank every member in the Optics Group of Experimental Facility Division, and Precision Mechanical Engineering Group of Instrumentation Development Division, NSRRC, for their kind assistance and friendship.


  1. 1.
    Ibrahim SR (1977) Random decrement technique for modal identification of structures. J Spacecraft Rickets 140:696–700CrossRefGoogle Scholar
  2. 2.
    Ibrahim SR, Mikulcik EC (1977) A method for the direct identification of vibration parameters from free response. Shock Vib Bull 47(Pt. 4):183–198Google Scholar
  3. 3.
    Asmussen JC, Ibrahim SR, Brincker R (1996) Random decrement and regression analysis of bridges traffic responses. Proceedings of the 14th International Modal Analysis Conference 1, 453–458Google Scholar
  4. 4.
    Vandiver JK, Dunwoody AB, Campbell RB, Cook MF (1982) A mathematical basis for the random decrement vibration signature analysis technique. ASME J Mech Design 104:307–313CrossRefGoogle Scholar
  5. 5.
    James GH, Carne TG, Lauffer JP (1995) The Natural Excitation Technique (NExT) for modal parameter extraction from operating structures. Modal Anal 10(4):260–277Google Scholar
  6. 6.
    James GH, Carne TG, Lauffer JP (1993) The natural excitation technique for modal parameter extraction from operating wind turbines. SAND92-1666. UC-261, Sandia National AboratoriesGoogle Scholar
  7. 7.
    James GH, Carne TG, Edmunds RS (1994) STARS Missile—modal analysis of first-flight data using the natural excitation technique, NExT. Proceedings of the 12th international model analysis conference. Honolulu, HI, 231–238Google Scholar
  8. 8.
    Newmark NM (1959) A method of computation for structural dynamics. J Eng Mech, ASCE, 85 (EM3) 67–94Google Scholar
  9. 9.
    Allemang RL, Brown DL (1983) A correlation coefficient for modal vector analysis. Proceedings of the first international modal analysis conference, society for experimental mechanics, Bethel, CT, 110–116Google Scholar

Copyright information

© The Society for Experimental Mechanics, Inc. 2015

Authors and Affiliations

  1. 1.Optics Group, Experimental Facility DivisionNational Synchrotron Radiation Research CenterHsinchuRepublic of China

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